By "oil sands" are meant mineable oil sands of the morphology found in the Athabasca deposits in Northern Alberta, and deposits elsewhere in the world having similar characteristics. To the extent that it may be possible to condition oil and sand combinations found elsewhere, naturally or as a result of oil spills, to have generally similar morphology, the invention is also applicable to such combinations. This morphology and some of its implications from the point of view of separating the bitumen content of the sand are discussed in a paper "Athabasca Mineable Oil Sands: The RTR/Gulf Extraction Process--Theoretical Model of Detachment", A Corti and M. Dente, Paper No. 81, Fourth UNITAR/UNDP Conference on Heavy crude and Tar Sands. A process and apparatus for practically implementing the theory set forth in this paper are described in Canadian Patents Nos. 1,165,712 of RTR Riotinto Til Holding S.A. and 1,167,238 of Ingeco International S.A., discussed further below. For the purposes of the present specification, the entities referred to variously as lumps, particles and matrices in the published art are referred to as granules, to distinguish them on the one hand from sand grains or particles which they contain, and on the other hand from large lumps of oil sand as mined. Such granules consist of a nucleus of sand grains covered with a film of connate water, which may itself contain fine particles, encapsulated, often with gas inclusions, within a layer of the heavy oil known as bitumen, which is essentially solid at ground temperatures. The terms oil and bitumen are used interchangeably in this specification.
The originally developed process for releasing bitumen from tar sands was the Clark hot water process, based on the work of Dr. K. A. Clark, and discussed in the above paper. The Clark process has disadvantages, some of which are discussed in the introductory passage of U.S. Pat. No. 4,946,597, notably a requirement for a large net input of thermal and mechanical energy, complex procedures for separating the released oil, and the generation of large quantities of sludge requiring indefinite storage.
Both the presently used commercial method and apparatus for the separation of oil or bitumen from oil sands based on the Clark process, and the similar process and apparatus described in U.S. Pat. No. 4,946,597, use vigorous mechanical agitation of the oil sands with water and caustic alkali to disrupt the granules and form a slurry, after which the slurry is passed to a separation tank for the flotation of the bitumen from which the bitumen is skimmed. As proposed in the U.S. patent, the process may be operated at ambient temperatures, with a conditioning agent being added to the slurry. Earlier methods, such as the Clark process, used temperatures of 85.degree. C. and above together with vigorous mechanical agitation, and are highly energy inefficient.
It is characteristic of both of the above processes that a great deal of mechanical energy is expended on physically disintegrating the oil sands structure and placing the resulting material in fluid suspension, this disintegration being followed by physical separation of the constituents of the suspension. Chemical adjuvants, particularly alkalis, are utilized to assist these processes. The separation process particularly is quite complex, as will be readily apparent from a study of U.S. Pat. No. 4,946,597, and certain phases have presented particularly intractable problems. Oil sands typically contain substantial but variable quantities of clay, and the very fine particles constituting this clay are dispersed during the process, limiting the degree to which the water utilized in the process can be recovered by flocculation of the clay particles. No economical means has been discovered of disposing of the flocculated and thickened clay particles, which form a sludge which must be stored in sludge ponds where it remains in a gel-like state indefinitely.
The Corti and Dente paper mentioned above suggests that better results should be obtained with a proper balance of mechanical action and heat application, and Canadian Patent No. 1,165,712 (the '712 patent) points out that more moderate mechanical action will reduce disaggregation of the clay content of the sands. Nevertheless, it continues to regard external mechanical action as playing an essential role in the disintegration of the oil and granules, which will inevitably result in partial dispersion of the clay.
Thus it proposes to use relatively more gentle agitation of the sand in a slowly rotating digester described in Canadian Patent No. 1,167,238 (the '238 patent). The digester in the '238 patent comprises in its broadest embodiment a shell, means for entry of liquids and solids into the shell at one end of the shell, a tubular outlet at the other end of the shell for discharge of liquids, a solids outlet at the same end as said liquids outlet, surrounding but separated from the liquids outlet, and a screw which surrounds the tubular liquids outlet to urge solids to and through the solids outlet, which screw is secured at its outer periphery to the shell. As seen in FIGS. 1, 2, 3 and 4 of the '238 patent, the operating embodiment of the digester includes numerous plates and bars secured to the shell for moving the solids along the shell, and a set of bars for separating the clay from the oil sands. Slurry is introduced at one end of the shell. This slurry is a mixture of oil sands and hot water. The slurry is moved by the plates, bars and screw down the shell during which it is agitated and the oil and water gradually separate from the solids. At the other end of the shell, such oil and water, together with some fine material that has separated from the solids, is removed from one central, axial outlet, while the solids exit the digester at its base.
This process, which is a cocurrent process, still requires considerable post digestion treatment, as described in the '712 patent. The post digestion steps include further separation of the liquids into an oil rich component and a middlings component consisting primarily of water and fines, removing the fines from the middlings component by flocculation and centrifuging, and further treating the oil rich component for the removal of contained water, fines and solids. A detailed outline of the process is described with reference to FIG. 1 of the '712 patent.